1. Field of the Invention
The present invention relates to an electroluminescent (EL) device comprising a transparent electrode layer made of a transparent conductive material having display electrodes in the form of segments that are selectively activated to represent desired characters, a back electrode located opposite to the transparent electrode layer, and an emissive layer, or phosphor layer, sandwiched between the transparent electrode layer and the back electrode.
2. Description of the Related Art
A so-called thin-film electroluminescent (TFEL) display is disclosed in Japanese Patent No. 2758785. That is, this display is fabricated by stacking a transparent electrode layer, an intermediate layer, an emissive layer (phosphor layer), and a back electrode located opposite at least to the transparent electrode layer on a transparent substrate. The transparent electrode layer forms display electrodes and conductive interconnects that are different in function. The intermediate layer is made of a dielectric material. The dielectric layer is made of a dielectric layer. A so-called organic electroluminescent (EL) device is disclosed, for example, in Japanese Patent Publication No. 32307/1994. This organic EL device is fabricated by stacking an anode, at least one organic layer, and a cathode of a metal conductive material on a transparent substrate. The organic layer consists either of an organic emissive layer or of a multilayer structure of organic layers such as a hole injection layer and, if necessary, a hole transport layer is added.
In such an electroluminescent (EL) device, an AC voltage of hundreds of volts is applied between a transparent electrode layer and a back electrode to excite a fluorescent material that is in the emission centers of the emissive layer. On returning to the ground state, the excited fluorescent material emits light.
The aforementioned transparent electrode layer is made of a transparent conductive material such as indium-tin oxide (ITO) that transmits light emitted from the emissive layer. The transparent electrode layer is partitioned into segments to represent desired shapes in terms of pixels by selective emission of the segments. Conductive interconnects are positioned from terminals to display electrodes. These conductive interconnects have resistances of tens of xcexa9/xe2x96xa1. Voltage drops occur across these conductive interconnects. If the conductive interconnects have different lengths, the voltage applied to each pixel becomes nonuniform, producing nonuniform emission. This impairs the display quality.
The back electrode described above is made of a conductive metal material such as aluminum that has a low resistivity and can be easily shaped into a film. Because of the film formation described above, the surface of the back electrode assumes a mirror-like state. Consequently, the surrounding scene may be seen by reflection off the back electrode. Again, this impairs the display quality.
In the aforementioned organic EL device, a DC voltage of several volts to tens of volts is applied between the anode and the cathode to cause the organic layer to emit light. The organic EL device has the advantage that it can be driven at a lower driving voltage than other kinds of EL devices.
The cathode of this organic EL device is made of a conductive metal material such as aluminum that has a low resistivity and can be easily shaped into a film. Because of the film formation described above, the surface of the cathode takes on a mirror-like state and the surrounding scene might be seen by reflection off the cathode. This deteriorates the display quality.
It is an object of the present invention to provide an EL device free of the foregoing problems.
This object is achieved by an electroluminescent (EL) device comprising a transparent substrate (1), a transparent electrode layer (2) formed on the substrate (1) and forming display electrodes (21) and conductive interconnects (23) that differ in function, at least one intermediate layer (4) made of a dielectric material formed on the transparent electrode layer (2), an emissive layer (5) formed on the intermediate layer (4), and a back electrode (6) formed on the emissive layer (5). The back electrode (6) is located opposite to the display electrodes (21). Auxiliary electrodes (3) made of a conductive metal material having a resistivity smaller than that of the material of the transparent electrode layer (2) are formed on the conductive interconnects (23).
Because the auxiliary electrodes (3) made of a conductive metal material having a resistivity smaller than that of the conductive interconnects (23) are formed on the conductive electrodes (23), voltage drops across the conductive interconnects (23) located between the terminals (22) and the display electrodes (21) are suppressed.
In one feature of the invention, the back electrode (6) and the auxiliary electrodes (3) are made of the same material (e.g., Cr or Al). This suppresses the aforementioned undesirable phenomenon, i.e., the auxiliary electrodes (3) are seen to float over the back electrode (6) when the EL device is viewed.
The above-described object is also achieved by an electroluminescent (EL) device fabricated by forming a transparent electrode layer (2) over the whole surface of a transparent substrate (1), forming an auxiliary electrode layer (3) over the whole surface of the transparent electrode layer (2), patterning the transparent electrode layer (2) and the auxiliary electrode layer (3) by etching to form display electrodes (21) and conductive interconnects (23) that differ in function, etching away the portions of auxiliary electrode layer (3) overlying the display electrodes (21), and then forming at least an emissive layer (5) and a back electrode (6) in turn on the remaining laminate.
This fabrication process provides an excellent etch selectivity and is straightforward. Voltage drops across the conductive interconnects (23) extending from the terminals (22) to the display electrodes (21) can be suppressed.
An electroluminescent (EL) device is also fabricated in accordance with the teachings of the invention by forming a transparent electrode layer (2) over the whole surface of a transparent substrate (1), forming a masking layer (31) of Cr over the whole surface of the transparent electrode layer (2), patterning the transparent electrode layer (2) and the masking layer (31) by etching to form display electrodes (21) protected by the masking layer (31), depositing aluminum (Al) on the display electrodes (21) to form conductive interconnects (23), removing the masking layer (31) on the display electrodes (21) differing in function from the display electrodes (21), and then forming at least an emissive layer (5) and a back electrode (6) of Al in turn. As a result, the undesired phenomenon, i.e., the auxiliary electrodes (3) are seen to float over the back electrode (6) when the EL device is viewed, is suppressed.
The present invention also provides an electroluminescent (EL) device comprising a transparent substrate (1), a transparent electrode layer (2) formed on the transparent substrate (1) and forming display electrodes (21) and conductive interconnects (23) that differ in function, a back electrode (6) formed opposite to the transparent electrode layer (2), an emissive layer (5) formed between the back electrode (6) and the transparent electrode layer (2), and terminals (22) formed by portions of the conductive interconnects (23) brought out toward the outer periphery of the transparent substrate (1). An auxiliary electrode layer (3) is formed on the transparent electrode layer (2) excluding the locations where the display electrodes (21) are formed so as to cover the whole surface of the transparent substrate (1) at least except for the locations where the display electrodes (21) are present.
When this EL device is viewed, the color of the auxiliary electrode layer (3) is imparted to all the portions excluding the display electrodes (21). Therefore, it is unlikely that the display electrodes (21), the conductive interconnects (23), and the other portions are seen to exhibit three different colors, respectively.
The present invention also provides an electroluminescent (EL) device comprising a transparent substrate (1), a transparent electrode layer (2) formed on the transparent substrate (1) and forming display electrodes (21) and conductive interconnects (23) that differ in function, a back electrode (6) formed opposite to the transparent electrode layer (2), an emissive layer (5) formed between the back electrode (6) and the transparent electrode layer (2), and terminals (22) formed by portions of the conductive interconnects (23) brought out toward the outer periphery of the transparent substrate (1). An auxiliary electrode layer (3) is formed on the transparent electrode layer (2) excluding locations where the display electrodes (21) are present. An intermediate layer (4) is interposed between the transparent electrode layer (2) or the auxiliary electrode layer (3) and the emissive layer (5). The intermediate layer (4) is made of a material having a color that has excellent ability to block light such as black. The intermediate layer (4) covers the whole surface of the transparent electrode layer (2) except for locations where the display electrodes (21) and the terminals (22) are present. When this EL device is viewed, the color of the auxiliary electrode layer (3) is imparted to all the portions excluding the display electrodes (21). Therefore, it is unlikely that the display electrodes (21), the conductive interconnects (23), and the other portions are seen to exhibit three different colors, respectively.
Furthermore, the present invention provides an electroluminescent (EL) device (100) comprising a transparent substrate (200), an anode (300) made of a transparent conductive material and formed on the substrate (200), at least one organic layer (400) formed on the anode (300), and a cathode (500) made of a conductive material and formed on the anode (300). The material of the cathode (500) has a low optical reflectance. A thin-film layer (600) is interposed between the cathode (500) and the organic layer (400) to promote electron injection from the cathode (500) into the organic layer (400). Consequently, the aforementioned undesired phenomenon, i.e., the surrounding scene is seen by reflection off the cathode (500) when the EL device (100) is viewed, can be suppressed; otherwise, the display quality would be impaired.
In one feature of the structure described above, the cathode (500) is made of carbon (C). In this case, it is relatively easy to form the film of the cathode (500). The process step for fabricating the cathode (500) is not complicated.
In another feature of the structure described above, the thin-film layer (600) is made of an electron transfer compound or an electron transfer material doped with a metal material having a low work function. As a consequence, the process step for forming the thin-film layer (600) is not complicated.
In addition, the present invention provides an electroluminescent (EL) device (100) comprising a transparent substrate (200), an anode (300) made of a transparent conductive material and formed on the substrate (200), at least one organic layer (400) formed on the anode (300), and a cathode (500) made of a conductive material and formed on the organic layer (400). The cathode (500) is made of a material having a low optical reflectance. An electron transport layer (610) made of an electron transport material and an electron injection layer (620) are stacked from the side of the organic layer (400) between the cathode (500) and the organic layer (400). The electron injection layer (620) is made of an electron transfer compound or an electron transport material doped with a metal material having a low work function. Consequently, the aforementioned undesired phenomenon, i.e., the surrounding scene is seen by reflection off the cathode (500) when the EL device (100) is viewed, can be suppressed; otherwise, the display quality would be impaired.
In one feature of the structure described above, the cathode (500) is made of carbon (C). In this case, it is relatively easy to form the film of the cathode (500). The process step for fabricating the cathode (500) is not complicated.
Other objects and features of the invention will appear in the course of the description thereof, which follows.